Seed crystal for epitaxial growth of single-crystal calcium fluoride

ABSTRACT

A nucleant seed for epitaxial growth of single-crystal CaF2 includes SrF2. In some embodiments, YF3, LaF3, or rare-earth fluoride is substituted into the SrF2 structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to processes for producing CaF₂ crystals. More specifically, the invention relates to a nucleant seed for epitaxial growth of single-crystal CaF₂.

2. Background Art

Single-crystal CaF₂ is commonly grown using the Bridgman-Stockbarger crystal growth process. For epitaxial growth of CaF₂, the process starts, as illustrated in FIG. 1, with a seed crystal 2 made of CaF₂ and having the desired crystallographic orientation. For deep-ultraviolet microlithography applications, for example, the desired crystallographic orientation is <111>, i.e., cubic (octahedral or cubic forms) crystal structure. The seed crystal 2 is placed at the base of a crucible 4. A starting material 6 comprising CaF₂ powder (or beads) is placed in the crucible 4, on top of the seed crystal 2. The crucible 4 is then placed in a vertical furnace 8 and heated to a temperature sufficient to melt the starting material 6. To prevent oxidation of the starting material 6 and the components of the furnace 8, the furnace 8 is typically maintained under vacuum and/or the process is carried out in an inert atmosphere.

After melting the starting material 6, the crucible 4 is moved downwardly at a predetermined rate (typically 0.3 to 5 mm/h), from a hot zone 10 into a cold zone 12. An insulating barrier 14 separates the hot zone 10 from the cold zone 12. FIG. 2 shows a typical temperature distribution along the vertical axis of the furnace (8 in FIG. 1). A single crystal of CaF₂ forms on the seed crystal (2 in FIG. 1) when the molten material reaches the zone 12 in which the furnace temperature is below the melting point of CaF₂. The CaF₂ crystal front propagates inside the crucible 4, within the material 6, as long as the crucible 4 is caused to move downwardly. The CaF₂ crystal conforms to the crystallographic orientation of the seed crystal 2 as it propagates inside the crucible 4.

To enhance the optical properties of the CaF₂ crystal, a scavenger is typically added to the starting material 6 to remove oxygen and hydroxyl ions. These impurities have been known to reduce transmission in the deep-ultraviolet region. The most common scavenger used is PbF₂. PbF₂ is solid and can be added directly to the starting material 6. Typically, a specific amount of PbF₂, typically 1 to 2% by weight, is mixed into the starting material 6. The mixture is then gradually heated to approximately 800° C. to 900° C., at which point PbF₂ reacts with the starting material 6 to form PbO. After the reaction is complete, the more volatile PbO is evaporated from the mixture by heating the mixture to the melting point of CaF₂ or higher. In an attempt to remove as much of the PbO as possible through volatization, the CaF₂ melt may become overheated and cause the seed crystal 2, which is also made of CaF₂, to completely melt and lose its crystallographic orientation.

SUMMARY OF THE INVENTION

In one aspect, the invention relates to a nucleant seed for epitaxial growth of single-crystal CaF₂ which comprises SrF₂. In some embodiments, a second fluoride is substituted in the SrF₂ structure, the second fluoride being selected from the group consisting of YF₃, LaF₃, rare-earth fluoride, and combinations thereof. In some embodiments, the rare-earth fluoride comprises one selected from the group consisting of YF₃, LaF₃, CeF₃, NdF₃, PrF₃, DyF₃, SmF₃, EuF₃, TbF₃, and GdF₃.

Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a Bridgman-Stockbarger crystal growth process.

FIG. 2 shows a temperature distribution along a vertical axis of the furnace shown in FIG. 1.

FIG. 3 shows a phase diagram for SrF₂ and LaF₃.

DETAILED DESCRIPTION

Embodiments of the invention provide a seed crystal for use in growing oriented, single-crystal CaF₂. The seed crystal is structurally similar to CaF₂ but has a higher melting point than CaF₂. In one embodiment of the invention, the seed crystal comprises SrF₂. There is crystallographic disregistry between SrF₂ and CaF₂, but this disregistry is well within the accepted values for effective nucleation of CaF₂ by SrF₂. In other embodiments of the invention, the seed crystal comprises a solid solution of SrF₂ and a fluoride such as LaF₃, YF₃, rare-earth fluorides, or combinations thereof. Examples of rare-earth fluorides suitable for use in the invention include, but are not limited to, CeF₃, NdF₃, PrF₃, DyF₃, SmF₃, EuF₃, TbF₃, and GdF₃. The effect of the fluoride substituted in the SrF₂ structure is to further increase the melting point of the seed crystal.

CaF₂ melts around 1415° C. The only other known fluoride phase with the same structure as CaF₂ and which has a melting point higher than CaF₂ is the strontium analog SrF₂. The melting point of this phase of SrF₂ is near 1455° C., about 40° C. higher than CaF₂. When pure SrF₂ is used as a seed crystal for the growth of CaF₂ crystal, the starting material (6 in FIG. 1) can be heated to much higher temperatures than the melting point of CaF₂ without melting the SrF₂ seed crystal. Of course, given enough time, that time being dependent upon kinetic factors, an essentially infinite reservoir of CaF₂ liquid would eventually dissolve even a refractory seed like SrF₂ or SrF₂—LaF₃ solid solution, but the time of survival will be longer than with a metable CaF₂ seed.

When a scavenger such as PbF₂ is added to the starting material (6 in FIG. 1), maximum removal of the by-products of the scavenging process can be removed via volatization without melting the SrF₂ seed crystal. Even if the SrF₂ seed crystal succumbs to dissolution in overheated CaF₂ melt, the process will be much slower than for melting or dissolution of a CaF₂ seed. Thus, removal of the by-products of the scavenging process can be completed before the seed crystal completely melts and loses its crystallographic orientation.

The melting point of the SrF₂ seed crystal can be increased by adding a fluoride, e.g., YF₃, LaF₃, rare-earth fluoride, or combinations thereof, to SrF₂. The strontium analog SrF₂ phase forms considerable solid solution, often up to 50 mole %, with YF₃, LaF₃, and rare-earth fluorides such as CeF₃, NdF₃, PrF₃, DyF₃, SmF₃, EuF₃, TbF₃, and GdF₃. The solid solution is formed by mixing molten SrF₂ with the fluoride and then cooling the mixture. The solid solution can be made to have a desired crystallographic orientation by cooling the mixture at a certain temperature. The fluoride gets substituted in the SrF₂ structure. The effect of these substitutions is usually to further increase the melting point of the SrF₂—CaF₂ solid solution. The melting point can be increased by 50° C. to 100° C. with LaF₃ and rare-earth substitutions in the 10 to 30 mole % range. The following table shows the melting point achieved by various fluoride substitutions in SrF₂.

TABLE 1 Melting Point for Solid Solutions of SrF₂ and rare-earth fluorides Seed Crystal Substitution in mole % Melting Point (° C.) Pure SrF₂ 1455 SrF₂—YF₃ 11 1460 SrF₂—LaF₃ 30 1550 SrF₂—CeF₃ 29 1550 SrF₂—NdF₃ 25 1535 SrF₂—PrF₃ 30 1540 SrF₂—DyF₃ 12 1490 SrF₂—SmF₃ 21 1525 SrF₂—EuF₃ 20 1510 SrF₂—TbF₃ 15 1500 SrF₂—GdF₃ 16 1520

The solid solutions shown in Table 1 above are expected to be effective as nucleant seeds for the epitaxial growth of CaF₂. Some small-ion rare-earths like Ho, Er, Yb, and Lu either lowered or did not increase the melting point of the seed crystal. It should be noted that LaF₃, YF₃, or rare-earth fluorides themselves are not appropriate as seed crystals because they are structurally dissimilar to CaF₂. Furthermore, the structure of the solid solution will become dissimilar to CaF₂ if too much fluoride is mixed into SrF₂. The amount of fluoride to be mixed into SrF₂ can be deduced from appropriate phase diagrams. See, for example, L. P. Cook and H. F. McMurdie, Eds., “Phase Diagrams for Ceramists,” vol. V11, FIGS. 7581-7987, American Ceramic Society, 1989. FIG. 3 shows a phase diagram for SrF₂ and LaF₃. The phase diagram shows that SrF₂ forms solid solution of up to approximately 47 mole % with LaF₃. The highest melting point of the SrF₂—LaF₃ solid solution occurs when LaF₃ substitution is about 30 mole %.

The invention provides general advantages. By using structurally similar but more refractory nucleant seed for growing the single-crystal CaF₂, maximum removal of the by-product of the scavenging process can be removed without completely melting the seed crystal.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims. 

What is claimed is:
 1. A nucleant seed for epitaxial growth of a single-crystal CaF₂ comprising SrF₂.
 2. A nucleant seed for epitaxial growth of a single crystal CaF₂ comprising a fluoride substituted in the SrF₂ structure, the fluoride selected from the group consisting of YF3, LaF₃, rare-earth fluoride, and combinations thereof.
 3. The nucleant seed of claim 2, wherein the rare-earth fluoride comprises one selected from the group consisting of CeF₃, NdF₃, PrF₃, DyF₃, SmF₃, EuF₃, TbF₃, and GdF₃.
 4. The nucleant seed of claim 3, wherein the fluoride substitutions in the SrF₂ structure is in a range from 10 to 30 mole %.
 5. A process for producing a single-crystal CaF₂ from a melt, comprising: contacting the melt with a seed comprising SrF₂; and moving the melt at a rate through a thermally-graded zone so that the single-crystal CaF₂ is grown on the seed.
 6. The process of claim 5, wherein the seed further comprises a fluoride substituted in the SrF₂ structure, the fluoride selected from the group consisting of YF₃, LaF₃, rare-earth fluoride, and combinations thereof.
 7. The process of claim 6, wherein the rare-earth fluoride comprises one selected from the group consisting of YF₃, LaF₃, CeF₃, NdF₃, PrF₃, DyF₃, SmF₃, EuF₃, TbF₃, and GdF₃.
 8. The process of claim 7, wherein the rare-earth substitutions in the SrF₂ is in a range from 10 to 30 mole %.
 9. A process for producing a single-crystal CaF₂ from a melt, comprising: contacting the melt with a seed having a composition SrF₂—X, where X is selected from the group consisting of YF3, LaF₃, rare-earth fluoride, and combinations thereof, and moving the melt at a rate through a thermally-graded zone so that the CaF₂ crystal is grown on the seed.
 10. The process of claim 9, wherein the rare-earth fluoride comprises one selected from the group consisting of YF₃, LaF₃, CeF₃, NdF₃, PrF₃, DyF₃, SmF₃, EuF₃, TbF₃and GdF₃.
 11. The process of claim 10, wherein the rare-earth substitutions in the SrF₂ is in a range from 10 to 30 mole %. 